Particle size analysis utilizing polarization intensity differential scattering

ABSTRACT

Two arrangements are disclosed to provide high resolution measurement of sub-micrometer and micrometer particle size distributions. In a first arrangement, scattered light is measured over a wide range of scattering angles. At the same time, light scattered at low scattering angles is measured with high angular resolution. In the second arrangement an improved Polarization Intensity Differential Scattering (PIDS) measurement is made possible by providing an interrogating light beam of selected wavelength including a first component parallel to the scattering plane and a second component perpendicular to the scattering plane. Photodetecting arrangements detect light scattered by the particles at least at two scattering angles.

BACKGROUND OF THE INVENTION

This invention relates generally to systems for measuring the size ofparticles, and, more particularly, relates to apparatus and methods forhigh resolution measurement of sub-micron and micron particle sizedistributions using polarization intensity differential scattering.

There are several prior art techniques for measuring the size anddistribution of sizes of particles in a sample by using lightscattering. Generally, to measure the sizes of individual particles, forexample, in a flowing stream of a liquid or gas, the particle-containingsample stream is illuminated by a constant light source and theintensity of light scattered by each particle is detected.

A particle scatters the light by an amount related to the particle size.In general, larger particles scatter more light than do smallerparticles. The relationship between the amount of scattering andparticle size can be determined either from theoretical calculations orthrough a calibration process. With a knowledge of this relationship,for a single particle at a time, the detected scattered light intensityprovides a direct measure of the particle size.

The distribution of particle sizes in a sample can be determined byindividually passing each particle in the sample, or a portion of thesample, through the scattered light detection apparatus, and tabulatingthe sizes of the various particles. In practice, this method isgenerally restricted to particles larger than 0.5 microns. Moreover,this method is relatively slow, because particles must be presented anddetected individually. This technique is referred to in the prior art asoptical particle counting.

Another prior art technique of particle sizing by light scattering isreferred to as static or "classical" light scattering. This method isbased upon illumination of a sample containing the particles to besized, followed by the measurement of the intensity of scattered lightat several predetermined angles. The intensity of light scattered from aparticle is a function of the size of the particle, the wavelength ofincident light, and the angle at which the scattered light is collectedrelative to the incident light. This method of particle sizing basedupon the angular dependence of the scattered light intensity can beemployed to determine the size distribution of a group of particles.

In particular, for particles larger than approximately one micron,scattering in the near-forward direction is well described by Fraunhoferdiffraction theory. The Fraunhofer principle yields the angulardistribution of scattered light in the focal plane of a lens. For agiven particle having a diameter d, the scattered intensity in theangular direction u is given by

I_(d), u =k₁ [(]d²)/4]² [2J₁ (k₂ du)/(k₂ du)]²

where

k₁ =a constant;

k₂ =]g; and

cJ(k₂ du)/=a Bessel function

The Fraunhofer theory, upon which most conventional laser diffractionsystems are based, shows that small particles diffract light at highangles while larger particles diffract light at smaller angles.Accordingly, by analyzing the composite diffraction pattern resultingfrom the scattering of monochromatic light beam by a given sample, andmeasuring the intensity of light scattered at predetermined angles, itis possible to deduce the size distribution of particles in the sample.This principle is widely used in laser diffraction methods andapparatus.

The following U.S. Pat. Nos. disclose examples of laser diffractionsystems for measurement of particle size:

    ______________________________________                                        3,646,652          Bol et al                                                  3,758,787          Sigrist                                                    3,809,478          Talbot                                                     3,873,206          Willcock                                                   4,017,186          Shofner et al                                              4,037,965          Weiss                                                      4,052,600          Wertheimer                                                 4,099,875          McMahon et al                                              4,167,335          Williams                                                   4,274,741          Cornillault                                                4,286,876          Hogg et al                                                 4,341,471          Hogg et al                                                 4,541,719          Wyatt                                                      4,595,291          Tatsuno                                                    4,648,715          Ford, Jr. et al                                            4,676,641          Bott                                                       4,679,939          Curry                                                      ______________________________________                                    

Certain prior art systems, among those disclosed in the above-identifiedU.S. Pat. Nos., utilize a single-optical-axis system, which may comprisea single lens or an assembly of lenses disposed along the same opticalaxis, for collecting forward scattered light--in a range ofapproximately 0.03-30.00 degrees from the axis of the incident beam--anddirecting the scattered light into 15-50 discrete detector cells, sothat each detector cell is illuminated by light scattered from theparticles at a particular scattering angle.

Other prior art systems utilize multiple collection lenses, disposedalong different optical axes, the lenses being optically connected to asingle photodetector via fiber optic or other optical coupling elements.

The configuration of a typical prior art laser diffraction instrument110 for particle size analysis is illustrated in FIG. 1. The beam from alaser 112 is expanded by a conventional beam expander assembly 114, inorder to cover a large number of particles in a sample and to reduce thedivergence of the beam. This parallel beam then passes through thesample 116, typically a dispersion contained in a sample holder 118. Thesample 116 can be stirred or pumped through the path of the laser beamin a re-circulation system, if the sample is a suspension or emulsion,or blown or sprayed through if the sample is a dry powder or spray.Light 122 scattered in the near-forward angles is collected by a Fouriertransform lens configuration 120 and directed toward a multicelldetector 124, arranged such that the position of a given particle in thepath of the laser beam does not affect the point at which the lightdiffracted by that particle falls on the detector 124.

Discrete detector segments or cells on the detector 124 sense theintensity of light corresponding to that scattered at different anglesto the incident beam. This intensity profile can be provided to acomputer 126 where digital processing elements determine the sizedistribution of the particles passing through the laser beam. Thecomputer 126 can be controlled by input from a keyboard 128, and canprovide data output via a display unit 130 and printer 132.

Diffraction-type particle size measurement instruments are widely usedfor measuring sample materials having a broad size distribution--i.e., awide size range--such as dust or pigment particles. Because largeparticles scatter light at small angles to the axis of an incident beam,and smaller particles scatter light at large angles, particle sizemeasuring systems utilizing scattered light detection must be capable ofmeasuring scattered light intensity over a large range of scatteringangles. Additionally, because large particles scatter light at smallangles, and relatively large changes in their size produce only smallchanges in scattering angles, it would be advantageous to measure lightscattered at small angles with relatively high resolution.

These two requirements pose conflicting demands on a single Fouriertransform lens or lens system, such as lens system 20 shown in FIG. 1.The result, in conventional diffraction instruments, is compromisedperformance at large angles, small angles, or both.

A limited range of measurement angles --approximately 20-60 degrees--canbe achieved in a conventional diffraction system for measurement ofparticle size, using a single Fourier lens and multiple detectors.Alternatively, a wide range of measurement angles, with sparse coveragewithin the angular measurement range, can be attained with multipledetectors, each coupled with a single lens or system of apertures todefine the scattering angle, or by moving a single detector successivelyto different scattering angles. Each of these approaches has beenimplemented in certain prior art devices, and each has significantlimitations.

In particular, when a single-axis optical system is utilized forcollecting scattered light over a wide angular range, a short focallength system can provide detection at large angles, at the cost ofcompressing low-angle scattered light near the optical axis, where itmay be obscured by laser spill-over or rendered unresolvable by thefinite size of detector segments.

When attempts are made to overcome these deficiencies by utilizinglenses of longer focal length, further problems arise, especially inmeasurement of scattered light at higher angles. In particular, thelongitudinal displacement of high angle detectors from the optical axiscan require large dimensions, resulting in a cumbersome instrumentpackage. The required displacement (R) of detectors from the opticalaxis is approximately

R=[FL]tan [theta]

where

[FL]=focal length of the lens

[theta]=scattering angle in the sample cell.

In addition, such a system would require large diameter lenses forcollecting light scattered at large angles, thereby increasing sphericalaberration and astigmatism and complicating the positioning of highangle detectors.

Systems which utilize elements for moving a single detector successivelyto different scattering angles are typically limited by low angularresolution, long measurement times and mechanical complexity.

Accordingly, it is an object of the invention to provide methods andapparatus for analysis of particle size based upon measurement ofscattered light, which enable the measurement of scattered light overwide angular ranges.

It is another object of the invention to provide methods and apparatusfor measuring particle size with high angular resolution at lowscattering angles.

It is a further object of the invention to provide particle sizeanalysis apparatus which is compact and mechanically reliable.

Moreover, although diffraction apparatus can be utilized to measureparticles in the 0.1-0.4 micrometers size range, the resolution ofconventional methods in this size regime is poor. The loss of resolutionis a consequence of the similarity in the angular pattern of scatteredlight of all particles which are smaller than, or roughly equal in sizeto, the wavelength of the illuminating light. Since the angularscattering patterns of all particles in this size range are similar,conventional methods are unable to reliably distinguish betweenparticles in this size range.

Another method to measure the sizes of particles in this size range isbased on a phenomenon involving the scattering by small particles oflight of different polarizations. For particles smaller than thewavelength of the incident light, for scattering at 90 degrees to thedirection of the interrogating beam, the light component having apolarization parallel to the scattering plane is scattered much lessefficiently that light polarized parallel to the scattering plane. Thescattering plane is defined herein as the plane containing the incidentlight beam and the line connecting the detector to the illuminated partof the sample.

This phenomenon, which is illustrated by the intensity vs. angle plot ofFIG. 8, is due to the transverse nature of light. In particular, theelectric and magnetic field oscillations which comprise a light beamoscillate in a direction perpendicular to the direction of propagationof the beam.

The difference in the observed intensity of 90 degree scattering light,for a first component of light polarized perpendicular to the scatteringplane and a second component of light polarized parallel to thescattering plane, is referred to herein as polarization intensitydifferential scattering (PIDS). PIDS has been used to measure sizes ofparticles in the 0.1-0.4 micrometers size range, and can be explainedwith reference to FIG. 9.

The abscissa on the graph of FIG. 9 represents particle diameternormalized by the wavelength of light. More particularly, the abscissais a variable conventionally called alpha, given by:

alpha=pi*d/lambda

where d is the Particle diameter, and lambda is the incident lightwavelength in the medium surrounding the particles. The ordinate in thegraph represents the photodetected PIDS signal per unit mass ofparticles. The PIDS signal, given by

PIDS=Iperpen,90-Ipara,90

is the unnormalized difference between the scattering intensity at 90degrees for incident light polarized perpendicular and parallel to thescattering plane.

The wavelength normalized diameter, alpha, is used on the abscissabecause all scattering phenomena are dependent on the ratio of particlesize to light wavelength, rather than on size alone. Thus the solid linein FIG. 9 can, for example, represent PIDS in the size range of 100 to1000 nm with light of wavelength 600 nm, or particles in the size rangeof 200 to 2000 nm with light of wavelength 1200 nm.

The large peak on the left hand side of the graph of FIG. 9 shows thatthe PIDS signal of particles below alpha=2 is the most significantsource of PIDS. This means that that PIDS is sensitive principally toparticles smaller than approximately 2/3 the wavelength of the incidentlight. If a series of PIDS measurements were made, each with light of adifferent incident light wavelength, a histogram of the particle sizedistribution could be produced as follows. The shortest wavelength PIDSmeasurement, for example, at 300 nm, would generally measure the mass ofparticles below approximately 200 nm. The next measurement might be madeat lambda=600 nm. This measurement would be sensitive to particlessmaller than approximately 400 nm. By subtracting the first measurementvalue from the second PIDS measurement value, the mass of particles inthe 200-400 nm range could be determined. The third PIDS measurementmight be made at 900 nm. By subtracting the second measurement valuefrom the third measurement value, the mass of particles in the sizerange 400-600 nm would be determined.

This process could be extended indefinitely toward larger or smallersizes, so long as sources of light of the proper wavelengths areavailable. In practice, however, conventional diffraction measurementsare more suitable for particle sizing above around 1 micrometer and theabsorption of UV light by quartz and silica limits the low rangeendpoint of wavelength to around 150 nm.

This conventional PIDS measurement technique, however, has severalsignificant deficiencies relating to resolution and accuracy. In ameasurement such as that described above, it has conventionally beenassumed that the PIDS value at any wavelength is sensitive almostexclusively to particles below a certain size. However, the secondary,smaller peaks toward the right hand side of FIG. 9 show thatconventional PIDS measurements have substantial response to particlesover a range of sizes. Thus a PIDS measurement at, for example, awavelength of 300 nm will actually be sensitive to a substantial portionof the mass of particles at sizes above 200 nm, with varying sensitivityto the various larger particles, as shown in FIG. 9. This lack ofdiscrimination means that conventional PIDS measurements are subject toserious artifacts and inaccuracies.

A useful "figure-of-merit" (FOM) for evaluating the discrimination of aPIDS measurement is simply the ratio of the area under the curve in themajor peak (small alpha value) to the total area under the first peakplus the areas under the subsequent smaller resonance peaks, typicallyout to five subsequent peaks. This FOM represents the ratio of thesensitivity to the particles of interest to the sensitivity to allparticles--including those not of interest. A FOM of 1 would be ideal; aFOM of 0 would mean that the method had no particle sizingdiscrimination whatsoever. As can be seen from the FIG. 9, the FOM ofthe conventionally measured PIDS in FIG. 9 is approximately 0.3,indicating a low value of size discrimination.

It is thus a further object of the invention to provide PIDS apparatusand methods having enhanced particle size discrimination.

Other general and specific objects of the invention will in part beobvious and will in part appear hereinafter.

SUMMARY OF THE INVENTION

The foregoing objects are attained by the invention, which provides PIDSmethods and apparatus for illuminating suspended particles in a samplecell along an interrogating axis with at least one interrogating lightbeam, having a selected wavelength, and including at least a firstcomponent polarized parallel to a scattering plane and a secondcomponent polarized perpendicular to the scattering plane. The inventionincludes photodetection methods and apparatus for detecting lightscattered by the suspended particles in a scattering plane at two ormore selected scattering angles.

One aspect of the invention includes photodetector elements forgenerating a first intensity signal representative of intensity ofscattered light corresponding to the first interrogating component, andelements for generating a second intensity signal representative ofintensity of scattered light corresponding to the second interrogatingcomponent. The signals are processed by an intensity differentialprocessing element, which executes selected arithmetic transformationsof the difference signal to generate a resultant signal representativeof the particle size distribution in the sample cell.

The invention can include an array of photodetectors disposed at anangular position substantially centered about 90 degrees with respect tothe interrogating axis.

The invention will next be described in connection with certainillustrated embodiments. However, it should be clear to those skilled inthe art that various modifications, additions and subtractions can bemade without departing from the spirit or scope of the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and objects of the invention,reference should be made to the following detailed description and theaccompanying drawings, in which:

FIG. 1 is a schematic diagram illustrating a conventional laserdiffraction apparatus for particle size analysis;

FIGS. 2A and 2B depict particle size analysis apparatus according to theinvention;

FIG. 3 is a schematic diagram depicting an embodiment of a photodetectorarray utilized in connection with the apparatus of FIG. 2;

FIGS. 4A and 4B depict another embodiment of photodetectors which can beemployed in the apparatus of FIG. 2;

FIGS. 5A and 5B depict a further example of photodetectors utilized inconnection with the invention;

FIG. 6 is a schematic diagram illustrating a lens configuration employedin accordance with the invention;

FIG. 7 depicts a practice of the invention utilizing a selected opticalaxis offset.

FIGS. 8 and 9 are graphs of measured intensity vs. angle and particlediameter, respectively;

FIG. 10 depicts PIDS values for particles of various sizes, measured atvarious angles;

FIG. 11 depicts a PIDS measurement system constructed in accordance withthe invention; and

FIGS. 12A and 12B depict a Projector module utilized in the embodimentof FIG. 11.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 2A depicts a particle size analysis system 2 constructed inaccordance with the invention, for measuring the size distribution ofparticles 4 suspended in a fluid 6 and contained in a sample cell 8. Inaccordance with conventional engineering practice, the walls 10 ofsample cell 8 can be constructed of a transparent material, such asglass or plastic, for admitting an interrogating beam 12 of light. Beam12 can be a substantially parallel beam of monochromatic light,generated by a conventional light source including a laser, spatialfilter, and collimator 14 of known design and construction. The beam 12is transmitted through the walls 10 of sample cell 8, where particles 4to be analyzed scatter a portion of the light 16 at various angles tothe axis of the beam 12. As discussed above, the amplitude and angle ofthe scattered light is in part a function of particle size.

The term "scattering angle", as defined herein, denotes the anglebetween the direction of propagation of the interrogating beam and thedirection of propagation of light scattered from the sample volume. Inparticular, a given portion of the scattered light will diverge from thedirection of propagation of the interrogating beam by an angle theta.

The unscattered portion 18 of the beam 12 passes out of the sample cell8 and into a set of optical components collectively referred to hereinas lower optical train 3. The unscattered beam 18 is transmitted througha lower collecting lens 17 which focuses the beam 18 to a point on amirror element 32 on the focal surface 20 defined by a photodetectorarray 22. Lens 17 can be a conventional Fourier lens constructed inaccordance with known engineering practice.

Focal surface 20 can be inclined toward the sample cell 8, so as to besubstantially tangent to the circumferential dashed line indicated inFIG. 2A. The focal surface 20 has associated therewith photodetectorarrays 22 and 24, described in greater detail hereinafter in connectionwith FIGS. 3-5. In accordance with the invention, focal surface 20 ispositioned so that the distance between the lens 17 and the focalsurface 20 is equal to the focal length of the lens 17.

A portion of the light 16 scattered by particles 4 in the sample cell 8also passes through the lens 17 and toward the focal surface 20. Thoseskilled in the art will appreciate that when focal surface 20 ispositioned so that the distance between the lens 17 and the focalsurface 20 is equal to the focal length of the lens 17, light scatteredat angle 02 to the input axis of the beam 12, at any location in thesample cell 8, will fall on approximately the same point 26 at the focalsurface 20. This is illustrated in FIG. 2A by exemplary light rays 28and 30.

As a result, each location on focal surface 20 receives light scatteredat a single, unique angle to the input axis of the beam 12 in the samplecell 8. By measuring light intensity at small discrete locations at thefocal surface 20, the intensity/angle profile of light scattered byparticles 4 in the sample cell 8 can be determined. Conventional lightscattering theory, such as Fraunhofer or Mie scattering, can then beapplied to determine the approximate particle size distribution in thesample cell 8, based upon this intensity/angle profile.

If the focal surface 20 is displaced forward or backward, theangle-mapping relationship between lens 17 and surface 20 no longerapplies. Rays scattered at the same angle, such as rays 28 and 30, nolonger fall on exactly the same point on surface 20, and a singledetector point 26 will collect a mixture of light from different angles,thereby providing a less precise measurement estimate of particle sizedistribution.

Referring again to FIG. 2A, light scattered at higher angles in thesample cell 8 passes into a second set of optical elements, referred toherein as the upper optical train, denoted collectively by referencenumeral 50 in FIG. 2A. The included light, illustrated by exemplary rays05 and 06, is transmitted through a collection lens 52 to a seconddetection surface 54. This detection surface 54 is related to itscollection lens 52 in the same manner that surface 20, in the loweroptical train, is related to its respective collection lens 17.Specifically, the detection surface 54 is displaced from the collectionlens 52 by a distance equal to the focal length of the lens 52.Preferably, the detection surface 54 is substantially spherical, ratherthan flat.

In the embodiment illustrated in FIG. 2A, the lowest angle of scatteredlight θ5 entering the upper optical train can be slightly smaller thanthe angle corresponding to θ4, and the highest angle of scattered lightθ6 entering the optical train can be several times greater. Thus, theoptical train 50 collects high angle light scatter over a wider anglethan does the lower optical train. Moreover, because the smallest angleθ5 is less than the largest angle θ4 measured by the lower optical train3, both optical trains measure a certain portion of light which isscattered at the same angles relative to the input axis defined by beam12. A first portion of scattered light is defined herein to have the"same" or "comparable" scattering angle as a second portion, when thefirst portion diverges from the interrogating beam by an angle of thesame magnitude as that of the second portion, and falls in the planedefined by the axis of the interrogating beam and the second portion.The overlapping angular ranges of the upper and lower optical trainspermit the illustrated apparatus to measure a continuous, uninterruptedintensity/angle profile in the angular range between θ1 to θ6.

The focal length of collection lens 52 can be, for example, 118millimeters, while that of collection lens 17 can be 2.5 times greater,or 293 millimeters. As a result, the detection surface 54 is 2.5 timescloser to its respective lens than is surface 20, and the light scatterfrom θ5 to θ6 is compressed along the detection surface 54 by a factorof 2.5 compared with that along the surface 20. These values areprovided solely by way of example, and other optical values can beutilized in accordance with the invention.

The invention has been described in connection with a "binocular"optical configuration utilizing two optical pathways for collectingscattered light and directing this light onto detector arrays. Thoseskilled in the art will recognize that the invention can beadvantageously practiced in connection with apparatus utilizing agreater number of optical pathways, providing the advantages ofmeasurement over a large angle and high resolution at small angles.

Moreover, the invention can be practiced in conjunction with a varietyof photodetector array configurations, including certain conventionalphotodetectors. In accordance with a preferred practice of theinvention, however, a low angle photodetector such as that depicted inFIG. 3 is utilized. The surface 20 has associated therewith a centralmirror element 32 and two silicon detector arrays 22 and 24. The innerdetector 22 is a monolithic silicon structure which can includesixty-two discrete silicon sectors 30 are substantially annular in form.As illustrated in FIG. 3, these sectors are disposed so as to radiateoutward from central mirror 32, which defines the position upon whichthe light source is focused.

The innermost sector 42 measures light scattered at small angles to thebeam 12, while the outer sectors 44 measure light scattered at greaterangles. The innermost sector 42 can be located slightly outside thepoint of spillover from the light beam. This spillover is an effect wellknown in the art, and results from small imperfections in the opticalelements In a preferred practice of the invention, sector 42 isconstructed as small as manufacturing practices permit, to achieveoptimum resolution. Thus, the radial width of the sector 42 can be, forexample, approximately 15 microns, while the arc length of sector 42 canbe approximately 50 microns. In the illustrated embodiment, the detectorsectors increase progressively in radial width and arc length, providingan exponential increase in the areas of the sectors.

Preferably, sectors proximate the mirror 32 can be quasi-symmetricallydisposed, to measure small, overlapping angular ranges of scatteredlight. Farther from mirror 32, the sectors can be symmetricallyarranged, for measuring comparable angles of scattered light.Additionally, a single row of sectors 44, referred to as a "tail,"measures light scattered at progressively higher angles. In theembodiment illustrated in FIG. 2A, for example, the axis A--A (FIG. 3)of detector 22 can be oriented vertically with the "tail" sectors 44extending upwards.

Moreover, in the embodiment illustrated in FIG. 2A, a second detectorarray 24 can be located on a vertical axis below detector 22. This array24 measures light scattered at higher angles to the incident beam 12than does detector 22. In particular, detector 22 measures scatteredbetween θ1 and θ2 while detector 24 measures between θ3 and θ4.

This orientation is further illustrated in FIG. 4, which provides afront view of detector arrays 22 and 24. FIG. 4 also indicates thatdetector array 24 is an assembly of two monolithic, linear siliconarrays. Detector array 24 can, for example, include sixteen sectors,each 1.58 millimeters long and 1.22 millimeters wide. The arrays fromwhich detector assembly 24 is constructed can include conventionaldetector elements manufactured by United Detector Technology, marketedas Part No. A2V-16.

In one embodiment of the invention, as indicated in FIG. 4, detectorarray 22 is asymmetrical, having tail sectors 44 extending outward onone side of central mirror 32. This configuration allows the innersectors 46 of detector 24 to be positioned proximate to the opticalcenter of mirror 32, and allows the innermost sectors 46 of detectorarray 24 to measure light scattered at angles near θ3 which arecomparable to those angles --i.e. θ2--measured by the outermost tailsectors 44 of detector 22. The optical overlap of sectors 44 and 46permits the particle size analysis system to measure a continuous,uninterrupted intensity profile from θ1 to θ4.

In accordance with this detector design, the employment of first sectorswhich increase exponentially in area, and other sectors which increaselinearly or are of constant area, reduces the dynamic range required ofassociated signal amplifiers and signal conversion electronics utilizedfor processing sector signals. These signals typically have a widedynamic range, and have heretofore posed significant difficulties in thedesign of signal processing electronics.

In particular, at small angles, where light scattered from largeparticles is of interest, observed light intensity falls off rapidlywith angle. In this region, exponentially increasing sectors tend toyield signals of the same order of magnitude, thus simplifyingsubsequent electronic processing. At high angles, observed lightintensity falls off in a substantially more linear manner. Accordingly,in this region, sectors which increase linearly in area, or which haveconstant area, tend to yield signals of the same order of magnitude,likewise reducing dynamic range requirements.

Additionally, at higher scattering angles, linear detectors have beenobserved to provide greater resolution. The resulting improved spectraldetail associated with smaller particles is advantageous in quantifyinganomalies associated with complex Mie light scattering effects, whichoccur when a particle size approaches the wavelength of interrogatinglight.

Detector geometry in accordance with the invention thus enablesincreased dynamic range of measurement and increased resolution, whilereducing the dynamic range required of associated signal processingelectronics.

In a further preferred embodiment of the invention, as illustrated inFIG. 4B, the detector arrays 22 and 24 are inclined inwardly, toward thecollection lens 17, in order to maintain the forward surface of thedetector sectors close to the surface 20. The surface 20 is preferably asubstantially spherical surface corresponding to the surface of bestfocus of the collection lens 17. Positioning the active face of thesectors of the detectors on or near this surface 20 maximizes focus, andminimizes the mixing of light scattered at different angles at eachsector's surface, thereby enhancing measurement precision.

The upper optical train detector arrays 56, corresponding to detectionsurface 54, are depicted in FIGS. 5A and 5B. In particular, FIG. 5Bindicates that the detector arrays 56 are inclined toward the opticalaxis 58 of collection lens 52, in order to better conform to thespherical nature of the detection surface 54. Detector arrays 56 caninclude two United Detector Technology, Part No. A2V-16 silicon arraysas described above in connection with FIG. 4. These detector arrays 56measure scattered light between θ5 and θ6 indicated in FIG. 2A.

The particle size analysis system of the invention can utilize a varietyof lens designs for collecting scattered light. The high anglecollection lens 52, as illustrated in FIG. 2A, can be a doublet, aconfiguration which provides a lens with a large aperture and shortfocal length. One configuration of the low angle lens 17 is asubstantially spherical lens, having three sides 60, and truncated asillustrated in FIGS. 2A and 6. In order to minimize beam spillover, theinterrogating beam is directed through the optical center of the lens.The sides of the lens perform no function, and thus can be removed. Inaddition, the upper portion of the lens 17 can be removed to facilitatepositioning of the upper collection lens 52 so that both lenses 17 and52 can collect light at comparable scattering angles from a givenilluminated sample volume.

Referring again to FIG. 2A, those skilled in the art will recognize thatthe upper lens 52 can be further displaced upward, and that lens 52 canbe positioned to collect light at angles as small as θ4. Displacement oflens 52 to this extent would preclude measurement over a comparableangular segment, but would provide a continuous measurement of theintensity profile to θ6. This configuration offers the advantages offurther reduced spatial requirements and a physically smaller instrumentpackage.

Alternatively, cutting of lens 17 can be avoided by directing theinterrogating light beam 18 to pass, not through the optical center ofthe lower lens 17, but through a region of lens 17 offset from theoptical center, as depicted in FIG. 7. This displacement permits upperlens 52 to be positioned for collecting light flux down to θ4, so that acontinuous measurement of intensity is obtained. In this alternativeembodiment, however, the focus of the beam 18 becomes non-optimal, andlaser spillover will be increased, thus reducing low angle sensitivity.

In a preferred embodiment of the invention, as indicated in FIG. 2A,collection lens 17 can be a "landscape" or plano-convex lens orientedwith its planar face facing the sample cell 8. Lenses of theplano-convex type ordinarily collect a substantially parallel bundle oflight from each distant element in a landscape, and direct each suchbundle onto a single position on a light-sensitive medium, such as a CCDarray or the film of a camera. In the particle size analysis system ofthe invention, the light of interest is composed of rays scattered fromthe sample cell 8 at substantially the same angle, forming a bundle ofparallel light. Because the precision of a diffraction instrument isimproved by focusing all light scattered at a given angle to a singlepoint, the plano-convex lens is advantageously utilized in theinvention.

This employment of the plano-convex lens is contrary to conventionalengineering practice. In conventional imaging devices, such asphotographic apparatus, the distance from the object to the lens is muchgreater than the distance between the lens and the image, and sharpimages are produced by orienting the planar surface of the lens towardthe image. If a plano-convex lens were utilized in this conventionalorientation in a diffraction instrument--i.e., with the planar surfacefacing the detector--the angular "blur" due to aberrations would beunacceptable. Designers of conventional diffraction instruments havetherefore concluded that plano-convex lenses are of limited utility insuch instruments, and have instead utilized expensive achromat lenses toobtain sufficiently sharp angular resolution.

We have found, however, that by positioning a suitable plano-convex lensin an orientation opposite that of the conventional orientation, angularresolution substantially equal to that attainable with an achromat canbe achieved, with a less expensive plano-convex lens. This unexpectedresult arises because the plano-convex lens is applied herein for adifferent purpose--i.e., as a Fourier lens in a diffraction instrument,rather than to form an image, as employed conventionally.

In accordance with the invention, therefore, the plano-convex lens canbe oriented with its planar surface facing the sample volume, and spacedsufficiently far from the sample volume so that spherical aberration andastigmatism are minimized. The configuration illustrated in FIG. 2Aoffers a combination of lens 17 and sample cell 8 offering minimum fieldcurvature and astigmatism, and thus maximum resolution.

One deficiency of certain conventional laser diffraction apparatus usedfor analysis of particle size is a reduction in resolution due tointerference by extraneous light. Thus, a preferred embodiment of theinvention utilizes a light trap or "beam dump" module. Referring toFIGS. 2A and 2B, the interrogating light beam 18 can be directed ontomirror 32 and reflected into a light trap 34. As shown in FIG. 2B, thelight trap 34 can be composed of two converging pieces of dark material36, such as glass, and preferably, a photodetector 38 for monitoring thepower of the reflected beam 18'. The converging angle of the glass 36can be approximately 9 degrees, an angle which causes the beam 18' tomake approximately seven reflections before re-emerging toward thesurface 20. Because the glass is optically coated, each reflectionreduces the intensity of the re-emerging beam 40 by a factor ofapproximately 10². Seven reflections therefore yield a 10¹⁴ reduction inthe power of the emerging beam 40. Accordingly, the emerging beam 40 hasan immeasurably small effect as it returns toward the surface 20.

The optically coated dark glass 36 can be, for example, three millimeterthick NGl glass manufactured by Schott Glass Technologies Inc. Thismaterial attenuates transmitted light by a factor of approximately 10⁶.In the light trap 34, where light passing into the glass 36 must passthrough it, strike a wall 37, and then re-emerge through the glass 36,the power of emerging light is therefore reduced by a factor in excessof 10¹².

The detector 38 can be, for example, a commercial silicon detector witha sensitive surface of about 0.4×0.8 inches in dimension. Such adetector is manufactured by EG & G Vactec Division. The detector 38 canbe positioned on one side of a dark glass element 37.

The invention thus provides a number of advantages over conventionalparticle size measurement methods and apparatus. As discussed above,certain conventional devices utilize a single optical train to providemeasurements over a wide angle of scattered light. In these devices, ashort focal length system permits detection to greater angles, butcompresses low angle scatter near the optical axis, where it may beobscured by laser spillover or rendered unresolvable by the finite sizeof a detection segment. Other conventional devices attempt to addressthis problem by employing a longer focal length lens, at the expense ofincreased spatial requirements, greater spherical aberration andastigmatism, and increased complication in positioning high angledetectors.

The multiple optical pathways of the invention offer the advantages ofcompactness and enhanced optical resolution. In addition, multiple lenstrains, even of the same focal length, offer economic advantages. Whilethe low angle lens train must be of high quality to minimize scatter andspillover--which reduces low angle resolution--the high angle lens trainneed not meet this requirement. This is because the interrogating lightbeam does not pass through the high angle lens train. This lens can bemanufactured to less stringent tolerances, and thus may be lessexpensive.

Moreover, lens fabricating costs are approximately proportional to lensarea, or the square of lens diameter. Replacing a single, large-diameterlens with two lenses of approximately half the diameter can reduce lenscost by approximately 50%.

In addition to these general advantages, the utilization of opticaltrains having different focal lengths permits a wide measurement span athigh angles, and high resolution at low angles. Different power opticaltrains permit the use of a common detector design for both high and lowresolution measurements, and allow greater flexibility in the selectionof detector arrays. The designer can select a low angle array withsector areas which grow exponentially to compensate for the rapid decayof intensity at high angles, and high angle arrays with sector areasthat are constant or increase linearly to compensate to the slow decayof intensity in this region.

While the apparatus described above in connection with FIGS. 2-7provides enhanced resolution in particle measurement, a preferredembodiment of the invention utilizes a novel polarization intensitydifferential scattering (PIDS) system to provide still greaterenhancements in accuracy and resolution.

We have found that the accuracy and discrimination of a PIDS measurementcan be substantially improved by using not only the PIDS data collectedat an angle of 90 degrees to the interrogating beam axis, as inconventional systems, but also the symmetry of PIDS scattering around 90degrees. The theoretical basis for this advantage can be explained withreference to FIG. 10, which shows PIDS values for particles of varioussizes, for scattering angles of fifty degrees on either side of 90degrees, measured with respect to the axis of propagation ofinterrogating light. As can be seen from FIG. 10, for small particles(i.e. particles having an alpha value less than 2), the pattern of PIDSaround ninety degrees is a symmetric, roughly quadratic curve with apeak at exactly 90 degrees.

Again referring to FIG. 10, for very small values of alpha, the PIDSpattern will have exactly the same shape, but the peak amplitude will belower because small particles produce less scatter per unit weight. Asparticle size increases above (alpha=2), the PIDS peak shifts towardlower angles and thus, although it is still roughly symmetrical, thecenter of symmetry shifts toward lower angles. Finally, for largerparticles, the PIDS curves acquire a shape similar to a cubiccurve--i.e., high amplitude at low angles, low at intermediate angles,followed by a local maximum, and then a decay at higher angles.

The data displayed in FIG. 9 could be generated from FIG. 10 by drawinga vertical line at 90 degrees, and plotting the value of the PIDS curvesfor the various alpha values shown. It should be noted that the smallerresonance peaks in FIG. 9 occur when the local maxima which occur atlarger alpha values, as in FIG. 10, line up at 90 degrees.

FIG. 10 thus illustrates that conventional PIDS measurements use only asmall part of the information potentially available from suchmeasurements. A 90 degree PIDS measurement is not responsive to theshape of the PIDS curve as a function of scattering angle. Moreover, asthe peak of the curve shifts toward lower angles--as alpha increasesfrom approximately two--a 90 degree PIDS measurement measures the PIDSoff-peak. The information, describing particle size, inherent in thepeak shift, which is caused by larger sized particles, is not sensed bya 90 degree PIDS measurement.

By measuring the PIDS at a number of angles around 90 degrees,information can be obtained about the shift of peak position, which ismanifested by a peak which becomes asymmetric about 90 degrees. As thePIDS curve increases its cubic character with still larger alpha values,this further information can be sensed with judiciously placeddetectors, as discussed in greater detail hereinafter.

The symmetry information obtained by multiple photodetectors whichmeasure PIDS values at angles between around 40-140 degrees increasesthe discrimination with which the invention can distinguish particles ofdifferent sizes.

In particular, the invention, in the embodiment illustrated in FIG.11,includes a PIDS module 2' including a projector element 210 whichproduces a collimated beam of light 212 characterized by one of aplurality of selectable wavelength and polarization combinations. Forexample, the light beam 212 can have any of three wavelengths of light,each polarized either perpendicular or parallel to the scattering plane.The PIDS apparatus can also include a sample cell 8' containing a streamof particles-to-be-measured, and a detector module 214.

Referring again to FIG. 11, the detector module 214 includes a pluralityof photodetector elements 201-205, which can be, for example,photodiodes, for measuring scattered light at selected scatteringangles, and a further detector 206 for monitoring the amplitude of theprojector beam at approximately zero degrees to the axis of the incidentbeam. In the illustrated embodiment, the photodetectors 201-205 aremounted on a bracket 219 having pinhole apertures 207-209, 211 and 213for admitting scattered light to detectors 201-205, respectively. Thefive photodiodes 201-205 are employed for detecting scattered light, anda sixth detector 206 is utilized for monitoring the projector beam 212'.Those skilled in the art will appreciate that a greater or lesser numberof photodetector elements may be employed.

In generating a PIDS measurement, the particles-to-be-measured aresuspended in a suitable liquid, such as water, to form a sample systemcontaining, for example, approximately 1.5 liters of liquid. This samplesystem is pumped continuously through the sample cell 8', at a pumpingrate which can be selected such that during each 10-30 second interval,all, or a substantial portion of, the 1.5 liters of particle-containingliquid passes through the sample cell 8' where the scattered lightmeasurement is made.

The PIDS measurement is made by sequentially projecting light of theselected wavelength/polarization combinations through the sample cell8', and measuring the average intensity of light scattered by each ofthe selected configurations of light. The light is measured at each ofthe scattered light detectors 201-205. In one embodiment, for each ofthe six wavelength/polarization combinations of light, the scatteredlight is measured for a time period of between 10 and 30 seconds. Thus,most of the particles in the 1.5 liter sample system are sensed witheach variety of projected light.

Detail of a projector module 210 for generating variouswavelength/polarization combinations is provided in FIGS. 12A and 12B.The projector module 210 utilizes a conventional light source 220, whichcan be a tungsten halogen element. The output of source 220 is collectedby condensor lens assembly 222, and passes through pinhole 224. Thelight beam is then modulated by a conventional bandpass filter 226,polarizer 228, and filter wheel 230. The orientation of polarizer 229and filter wheel 230 can be selected, in accordance with knownengineering practice, to provide varying polarization and filteringoutputs. Projection lens 232 collects the polarized, filtered light anddirects it toward the sample area 234.

Referring again to FIG. 11, a conventional microprocessor orcomputational device 126 can process the average scattered lightintensities measured by detectors 201-205, together with informationfrom detector 206 and projector module 210, to generate particle sizedistribution data. In one embodiment of the invention, for eachwavelength of light, the average scattered intensity resulting fromincident light polarized parallel to the scattering plane can besubtracted from that resulting from incident light polarizedperpendicular to the scattering plane. The subtraction is performed foreach of the five detectors, as described by the following expression:

I(lambda, theta₋₋ i)=I(lambda, theta₋₋ i, perpen)-I(lambda, theta₋₋ i,para)

where i=1, 5 denotes the five detectors 201-205 located at five selectedscattering angles in an angular range substantially centered atapproximately 90 degrees; lambda is the wavelength of the incidentlight; and theta is the scattering angle, defined in accordance withconventional engineering practice. The I(. . . ) terms denote thevarious average scattered intensities. In accordance with the invention,a function I(lambda₋₋ j,theta) is generated for each wavelength,lambda₋₋ j. The beam strength monitor photodetector 206 (FIG. 11) isused to normalize the above signals for changes in the incident beamintensity.

We have found that the symmetries of the three I(lambda₋₋ j, theta)terms about theta=90 degrees is characteristic of particle size forparticles with diameters approximately equal to lambda (see FIG. 10). Inparticular, a particle size distribution can be extracted from the threeterms I(lambda₋₋ j, theta), j=1, 3 by matching the pattern of themeasured I(lambda₋₋ j, theta) to the pattern of I(lambda₋₋ j, theta)calculated for particles of various sizes in the desired range. Forexample, using an apparatus such as that described above in connectionwith FIG. 11, the sizes of four polystyrene latex beads of known sizeswere measured. The results are shown below in Table 1.

                  TABLE 1                                                         ______________________________________                                        PIDS Measured Sizes of Polystyrene Latex Standards                            Nominal Size                                                                  Diameter .109 um   .175 um   .246 um .305 um                                  (um)     Volume %  Volume %  Volume %                                                                              Volume %                                 ______________________________________                                        .08                                                                           .09                                                                           .11      93%                                                                  .13       7%                                                                  .15                100%                                                       .21                          82%                                              .24                          18%     95%                                      .28                                   5%                                      .33                                                                           .39                                                                           .46                                                                           Meas.    .11 um    .15 um    .22 um  .24 um                                   Size                                                                          ______________________________________                                    

Apparatus employed for these measurements can utilize light ofwavelengths centered at lambda=450, 600 and 900 nm, using conventionalfilters, such as filter wheel 230 of FIG. 12, which transmit light ofapproximately 40 nm to either side of the nominal wavelength center, andscattering angles of 70.5, 81.4, 90, 98.6 and 109.5 degrees. Aconventional microcomputer 126 (FIG. 11) or other processing device,programmed in accordance with known pattern matching algorithms, cancalculate the relative volumes of particles in, for example, twelve sizecategories, geometrically spaced, for example, between 0.08 and 0.5micrometers.

Those skilled in the art will appreciate that the apparatus can beconstructed to employ more or fewer scattering angles and wavelengths oflight. The significant feature of the invention is the evaluation of thesymmetry of the polarization difference signal about the 90 degreesscattering angle, this angle being measured with respect to theinterrogating beam incident upon the sample cell. In order to measurethe symmetry, a minimum of three scattering angles and two lightwavelengths should be evaluated.

The invention has important advantages over conventional lightscattering methods, including certain conventional laser diffractiondevices. One advantage is that measurement accuracy is relativelyinsensitive to the often unknown refractive index of theparticles-to-be-measured. The invention also has the advantage, overconventional polarization dependent sizing methods, of enhanceddiscrimination in rejecting effects of particles outside the desiredmeasurement range. In particular, in conventional polarization dependentsizing methods, particles outside of, but adjacent to, the measurementrange will distort the measured size distribution within the measurementrange. The method of the invention is far less susceptible to suchanomalies, and provides enhanced resolution and accuracy in particlesize analysis.

It will thus be seen that the invention efficiently attains the objectsset forth above, among those made apparent from the precedingdescription.

It will be understood that changes may be made in the above constructionand in the foregoing sequences of operation without departing from thescope of the invention. It is accordingly intended that all mattercontained in the above description or shown in the accompanying drawingsbe interpreted as illustrative rather than in a limiting sense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention asdescribed herein, and all statements of the scope of the inventionwhich, as a matter of language, might be said to fall therebetween.

Having described the invention, what is claimed as new and secured byLetters Patent is:
 1. A system for measuring the size of particlessuspended in a sample cell, comprisingillumination means forilluminating the sample cell along a first interrogating axis with oneor more interrogating light beams, each characterized by a selectedwavelength and including at least a first interrogating component havinga polarization parallel to a scattering plane and a second interrogatingcomponent having a polarization perpendicular to said scattering plane,photodetector means for detecting light scattered by said suspendedparticles for at least one selected wavelength and in a scattering planeat at least two selected scattering angles, said photodetector meansincluding means for generating a first intensity signal representativeof an intensity of scattered light corresponding to said firstinterrogating component, said photodetector means including means forgenerating a second intensity signal representative of an intensity ofscattered light corresponding to said second interrogating component,and intensity differential processing means, coupled to saidphotodetector means, for generating a signal representative of theparticle size distribution in said sample cell for at least one selectedwavelength, said intensity differential processing means including meansfor generating a difference signal representative of the difference ofsaid first and second intensity signals, said intensity differentialprocessing means including calculation means for generating a resultantsignal representative of a selected arithmetic transformation of saiddifference signal.
 2. A system according to claim 1, wherein saidcalculation means includes means for generating a symmetry signalrepresentative of symmetry of said difference signal values about 90degrees of said interrogating axis.
 3. A system according to claim 2,wherein said photodetector means includes a plurality of photodetectingelements disposed at selected angles about said sample cell.
 4. A systemaccording to claim 3, wherein said photodetector means includes an arrayof photodetecting elements disposed at a position centered about 90degrees of said interrogating axis.
 5. A system according to claim 4,wherein said intensity differential processing means includes controlmeans for generating a plurality of said distribution-representativesignals, each corresponding to a selected wavelength of saidinterrogating light beam.